WASTEWATER
By Rick Lallish
What is the most common type of lagoon in use to treat municipal wastewater?
A. Anaerobic lagoon
B. Aerated lagoon
C. Polishing lagoon
D. Facultative lagoon
Answer: D. The most common lagoon treatment system is the facultative lagoon. These lagoons are typically 3 to 8 feet deep. They have dual layers: an upper aerobic layer and bottom anaerobic layer. The algae in the aerobic layer supply dissolved oxygen. Light penetration determines the aerobic layer’s depth. The waste byproducts from the aerobic layer trickle down to the anaerobic layer, where digestion takes place. The layers supplement each other. Controlled discharges from these lagoons may provide detention times of up to 180 days. More information may be found in the Office of Water Programs California State University, Sacramento textbook: Operation of Wastewater Treatment Plants, volume one, eighth edition, Chapter 8.
DRINKING WATER
By Drew Hoelscher
What is the purpose of a rate-of-flow controller on a gravity filter?
A. Gradually close as the headloss increases during a filter run
B. Gradually close as the headloss decreases during a filter run
C. Gradually open as the headloss increases during a filter run
D. Gradually open as the headloss decreases during a filter run
Answer: C. During a filter run, the water depth above the media remains constant due to the rate-of-flow controller on the filter effluent piping. As the filter run hours increase, the media becomes more clogged, resulting in higher headloss. As the headloss increases, the rate-of-flow controller gradually opens so that constant depth of water above the media is maintained. In other words, a rate-of-flow controller is mostly closed when headloss is low and is mostly opened when headloss is high. The filter should be backwashed when the rate-of-flow controller is completely open, because at that point, future increases in headloss cannot be compensated for.
About the authors
Rick Lallish is water pollution control program director and Drew Hoelscher is program director of drinking water operations at the Environmental Resources Training Center of Southern Illinois University Edwardsville.
